(continued) .... I finished up the day by shooting a couple of the WC297 loads into the water tank. There was absolutely no yawing, but both bullets had some funny gas cut etching on the bottom band.

A closeup of the gas cutting. On this particular bullet, it was all the way around the base, so I'm pretty sure it was not due to water damage. And I've seen spots of similar etching on other bullets, so I'm getting pretty confident that is is gas cutting, albeit mild gas cutting.

Observations and Lessons Learned:-- the jury is still out on the snug slip fit freebore. It groups OK horizontally, but the velocity variation is killing the vertical spread.-- leading is definitely a problem with the plain base bullets. I can't explain why the original "bad" chamber didn't lead, I'm out of ideas to fix the leading, and it's never going to shoot satisfactorily with this leading, so I think I need to admit defeat at least for the moment and retreat to gas check bullets. Perhaps I'll revisit the plain base bullets later if I get new ideas to try.-- the velocity variation is the other big problem. Things that contribute to velocity variation: 1) chambering resistance, a known problem for Contenders 2) most of the bullet is outside the case, so the case can't get a good grip 3) no crimp and 4) hard-to-ignite powders.

Things to Try Next Time-- a 170 gr. gas check bullet. Heavier =longer= more bullet in the case for a better grip.-- try to crimp in a lube groove-- keep tweaking seating depth-- keep trying different diameters. It may sound counterintuitive, but Contenders usually shoot best with a cartridge that chambers effortlessly. -- if I do continue playing with plain base, try walking the velocity down until the leading goes away.

I lightly polished the entrance of the freebore with fine emory, and remeasured the freebore several different ways, trying to make sure the diameter is what I think it is.

A 0.3590" gage pin will pass through the freebore, though it's a snug fit with moderate friction toward the end.

A 0.3596" gage pin will only go about 1/3 of the way into the freebore.

A tri-mic reads 0.3601" at the entrance of the freebore.A tri-mic reads 0.3597" in the middle of the freebore.A tri-mic reads 0.3587" at the far end of the freebore.

The tri-mic is the "gold standard" for ID measurement, so I have confidence in its measurements.

On the other hand, the "feel" of a gage pin's fit is similar to the "feel" of a bullet's fit. So for practical purposes, the gage pin fit is probably what I should go by, if I want the bullet to be a "snug slip fit."

Conclusion: a 0.359" bullet will be a very snug fit, with a little bit of friction. For a true slip fit the bullet probably needs to be 0.3585."

I also spent some time studying the barrel carefully with a borescope, and rechecking again with the borescope after each cleaning step.

The corners of the grooves are square, but where leading has occurred there was significant lead in the corners. This may explain why the engraving appears to be rounded rather than square on bullets recovered from the water tank.

My usual cleaning procedures, include liberal use of JB bore paste, would remove 80% of the lead, and make the barrel appear sparkling clean to the naked eye, however in the bore scope you could still see significant lead in the corner of the grooves.

Conclusion: leading has definitely been a problem. Even when the barrel appears clean to the naked eye, there may be enough lead in the corners of the groove to gouge the bullet and promote gas cutting (this lesson may also apply to plain base bullets in my 6x45 barrel?).

Eagle View Arms sent me a new bolt at no charge, to replace the broken bolt.

Today we tried a new 175 gr. gas check spitzer. The hope for the 175 gr. gas check bullet was 1) to reduce the awful velocity variation and 2) to reduce leading.

Note the measurements for the jam fit -- no two measurements were exactly the same! If you pushed a little harder, the bullet would go in a little further. There was no definite, repeatable jam point. That is part of the reason this throat struggles with velocity variation.

All of today's loads used WC297 powder, oven treated WW at ~25 BHN, sized 0.359", and Win Mag primers. The bullet was nose sized slightly in a 0.359" x 0.358" die, just enough to size the front band to 0.3585". Because that happened to result in the seating depth that I wanted.

Vertical stringing was not as horrible as last week, yet every group had one or two vertical outliers that correlated to their velocity.

Today's big surprise was that all of today's loads with the gas check bullet leaded. Perhaps not as much as a plain base bullet, but still significant leading in the usual spot about 4" from the breach. Now let me say that the barrel looked decent to the naked eye, so if I had not checked with a bore scope, or if I had not performed the tight fitting patch test, I might not have been aware that there was a leading problem.

Specs of lead on a tight fitting patch after one of today's groups. This was a very tight patch, requiring a lot of effort to push through the barrel. The tight patch test is my "gold standard" for judging leading.

Observations, Conclusions, and Questions-- why didn't the gas check fix the leading? -- there was no significant difference in accuracy or leading between today's 4 loads.-- there was a significant difference in velocity standard deviation, ranging from 1.07% to 2.03%.-- today's GC loads were not necessarily more accurate than previous PB loads, but they were more consistent, probably due to more consistent barrel condition.

Things to Try-- continue experimenting with the jam fit, even though the sweet spot seems to be from 0.015" off to 0.010" in. -- try sizing the bullet 0.360" to reduce leading, though it may run out of clearance at the cartridge neck.-- try sizing the nose even more for an easier fit.-- try walking the velocity down. It may be that I am simply pushing these bullets too fast and at too high a pressure (Quickload believes today's loads were 37,000 - 40,000 psi). Yes, other cartridges can successfully drive cast bullets faster and at higher pressures, but they typically use slower powders that are gentler on cast bullets. The 357 mag uses relatively fast pistol powders that reach peak pressure after only 0.5" - 0.6" of bullet travel, if you believe Quickload.

-- revisit powder coating, or other coatings. My previous experiments coating were not impressive, but it may be that my technique was flawed.-- revisit soft checks. -- long term, if this throat doesn't work out, I'm leaning toward rechambering to 357 Herrett, not to get higher velocity, but to get the same velocity with slower, gentler powders like 4198 and 4895.

The load was:-- 17.5 gr. WC297-- Win Mag primer-- 155 gr. bevel base sized 0.359" and oven treated to ~25 BHN before painting the base-- the paint increased the diameter of the bottom band to 0.360" - 0.361"-- the oversize bottom band made a few of the cartridges difficult to chamber-- 1.860" COL-- seated right at the jam point-- 1963 fps-- 1.2% velocity standard deviation-- 2.40", 1.97" groups-- 0.68" mean radius

Nothing to brag about, but this is the best accuracy so far for the "snug slip fit freebore."

The Taran overlay of both groups into a single 20-shot group. Taran terminology:R50 = mean radiusD5X = predicted size of 5 shot group, based on the mean radiusD10X= predicted size of 10 shot group

Taran is pretty cool. My only complaint is that it does not calculate Radial Standard Deviation.

Judging leading is always subjective. To the naked eye, the painted bases leaded less than unpainted bases. A bore scope showed that there was a little bit of leading, though not as bad as unpainted bases, and not even as bad as yesterday's gas check bullets. A tight fitting patch did show some specs of lead, though not a lot. In summary, the painted bases reduced leading but did not completely eliminate leading.

Conclusions, Observations, and Questions:-- the paint on the base does seem to reduce gas cutting and leading. My guess is that it acts as a physical barrier, insulating the lead from the hot gases. -- did the fact that the paint increased the diameter of the bottom band to 0.360"+ play a role ? -- it seems like most of the benefit of painting/coating is achieved by painting/coating only the base. -- accuracy was halfway decent even though the load was just a guess with no tweaking. This is definitely worth pursuing further.-- If I'm going to shoot 0.360" bullets in this chamber, the case neck thickness need to be uniformed for reliable chambering.

Things To Try Next Time:-- uniform the case neck thickness-- try unpainted 0.360" bullets-- try painted bases in other cartridges, like the 6x45 Contender, to see if the trend continues

Conclusions, Observations, and Questions-- Taran says there is no significant difference between today's painted bases vs. Hi-Tek. Though neither was acceptable.-- why didn't the painted bases shoot as well as last time?-- one variable is the nose-sizing step. There is no 100% repeatable way to set up the nose die, so each batch turns out a little different and has a slighty different jam point. Today's bullets seated out 0.040" further than last week's bullets. -- Hi-Tek definitely eliminated leading, but the jury is out on the 385 F quenching. -- note that we didn't experiment with seating depth, powder charge, etc.. For now we're trying to use the same load to test these different coatings.

Things To Try Next Time-- Hi-Tek with no lube-- Hi-Tek with no quench-- epoxy paint on the whole bullet not just the base-- try to set nose-sizing so that jam point is at 1.860" COL.-- keep trying different coatings. The coatings definitely reduce base melting & gas cutting, so it's just a question of finding an affordable coating that can tolerate 470 F heat treatment.

Light fouling after polycryclic loads, 4" from breach. This is the spot where it fouled the most with uncoated bullets. This is an improvement compared to the uncoated bullets.

After polycrylic loads, 6" from breach. This is actually worse compared to uncoated bullets.

After polycrylic loads, 8" from breach. This spot would have been perfectly clean with uncoated bullets & HVR, so this is definitely a step backwards. The fact that the polycrylic & 45/45/10 fouling gets worse further down the barrel suggests that it was "running out of lube."

Today's target.

Observations, Questions, and Conclusions:-- I nose sized all the bullets with the same setting, hoping that would result in the same jam point, but because of the 3 different hardnesses -- 11, 15, and 25 BHN -- I ended up with a different jam point for each hardness. Drats. -- 20 shots is not enough to prove much about accuracy other than to weed out the "bad" combinations. -- but 20 shots is enough to prove something about velocity and standard deviation.-- the hard bullet had lower standard deviation than the soft bullet, prolly because it has more engraving resistance that helps the powder burn.-- Hi-Tek & Ben's Liquid Lube was the cleanest by far.-- polycrylic & 45/45/10 was the least clean.-- both polycrylic & urethane fouled lightly 6" - 8" beyond the breach, hinting they may have been "running out of lube." -- apparently tumble lube is enough to prevent polymer fouling, however I suspect accuracy and velocity would be more consistent with "real" lube. Even though coated bullets don't require much lube to prevent fouling, the lube may still help provide a good gas seal and minimize velocity variation. -- the jury is still out of on hard coated bullets vs. soft coated bullets. In general, coated bullets do not seem to be very sensitive to bullet hardness.

Things to Try Next Time-- I'm still inclined to make urethane shake-n-dry my standard coating, at least until I find a better coating.-- shootout between Ben's Liquid Lube, Rooster Jacket, and HVR